The purpose of these studies is to attempt to define sites and mechanisms of altered renal function in experimental models in the rat which have reasonably similar clinical counterparts. Clearance, micropuncture and microdissection techniques will be utilized to study renal sodium retention in (1) a form of nephrotic syndrome in which glomerular filtration rate is normal and (2) after adrenal enucleation, a condition which leads initially to sodium retention and later to hypertension. Similar techniques will be used to study a model which may bear on the pathogenesis of acute renal failure. Renal hemodynamics will be studied during and after partial or complete interruption of renal blood flow in the rat, with special reference to the action of mannitol.